Here is how to deal with tunnels.

Listen for digging noise.

Look on map, if no teammates are nearby YOU HAVE SOME MOLES!

Go to where the digging sounds loudest and listen while standing still to hear where those vermin are headed to.

Start digging straight down an the area they JUST dug past, making sure to periodically dig to the left and right in case you weren't directly over the tunnel

If all goes well you will drop down into the giant stench filled hole those cowardly rats were digging, and then can EASILY shoot those (insert color here)scum in the backs like they deserve!

What helps is digging a massive trench around your intel tower extending to the bottom of the map.

There is no valid counter to tunneling.

This is because finding a tunnel and/or shooting diggers doesn't actually do anything to the tunnel itself. That which has been dug is not affected at all, and any further digging will be easily continued.

Shooting tunnelers is an inconvenience for the tunneling team. Having tunnels underneath your territory is a massive disadvantage even if you know about them.

Wow, semi-ninja'd by Jacob. That trench will probably take longer to make than that tunnel the enemy is digging across the map. Unless you're Green. Green always gets it easy concerning intel defence, for some reason.